Cytotoxic CD8+ T-cells play a key role in host defense by killing virally-infected or malignancy-altered cells. They perform this function by binding to specific peptides (epitopes) presented in the context of MHC class I molecules. These epitopes must meet stringent length (8-10 amino acids) and sequence requirements in order to successfully bind MHC class I. Much of the necessary processing activity has been attributed to the 26S proteasome, an enzymatic complex located in the cytoplasm. C-terminal trimming of peptides, in particular, is generally accepted to be exclusively the domain of the proteasome. However, our preliminary studies demonstrate that C-terminal extensions of certain peptides can be trimmed in the cytosol, generating the active epitope, in the presence of proteasome inhibitor. This indicates that another enzymatic activity in the cytosol contributes to C-terminal peptide processing. This possibility opens up a whole new understanding of a critical immunologic pathway. Our long term goal is to gain a better understanding of this alternative pathway and, ultimately, to apply this new knowledge to areas such as vaccine development and tumor immunology. The immediate goal is to identify the source of this activity and confirm its role in antigen processing. This will be accomplished through conventional protein purification techniques guided by in vitro presentation assays, state of the art protein identification techniques and, ultimately, siRNA knockdown experiments. NARRATIVE: Cytotoxic T cells of the immune system can play a critical role in eliminating infections and in attacking cancer cells. They identify the cells that they are to kill by the peptides that are displayed at the surfaces of their "targets". The research proposed here focuses on how the peptides are generated within the cell. The better we understand peptide production (also termed "antigen processing"), the more control we will have over the recruitment of this important cell type of the immune system.